Free-roaming house cats kill an estimated 4 billion wild animals across the U.S. every year, including birds, mammals, reptiles and amphibians.

Cat owners often wonder about their pets' secret outdoor lives, but few are curious enough to actually follow them around the neighborhood. And thanks to a new study by the University of Georgia and National Geographic, that isn't necessary: Researchers attached video cameras to 60 house cats that are allowed outside, hoping to learn how free-roaming felines spend their free time.

The answer? About a third of the pet cats killed time by killing wildlife.

That may not surprise cat owners who regularly find tiny corpses on their doorsteps, but the study suggests house cats kill even more prolifically than many people realize. The researchers found the cats that killed did so about 2.1 times every week they spent outside, but brought home fewer than 25 percent of their kills. That could mean U.S. cats kill more than the previous estimate of 1 billion native birds and other animals every year — possibly as many as 4 billion.

"The results were certainly surprising, if not startling," says UGA researcher and lead author Kerrie Anne Loyd. "In Athens-Clarke County, we found that about 30 percent of the sampled cats were successful in capturing and killing prey, and that those cats averaged about one kill for every 17 hours outdoors, or 2.1 kills per week. It was also surprising to learn that cats only brought 23 percent of their kills back to a residence."

Working with National Geographic's Remote Imaging Department, Loyd and her colleagues attached lightweight video cameras (known as Crittercams, or "KittyCams" in this case) to 60 outdoor house cats in Athens, Ga. The cats' owners volunteered for the study by answering ads in local newspapers, and downloaded footage from the cameras at the end of each recording day. The study extended through all four seasons, and Loyd says the cats averaged five to six hours outside daily.

The cats killed a wide range of wild animals, including lizards, voles, chipmunks, birds, frogs and snakes (see the graph below). The study didn't include feral cats, but previous research suggests ownerless felines are at least as deadly as their more coddled cousins. A 2010 study by the University of Nebraska, for example, found that feral cats have driven 33 bird species to extinction worldwide, and that they prey more on native than non-native wildlife. In fact, since domesticated cats aren't native to North America, this leads some wildlife advocates to consider cats an invasive species themselves, on par with kudzu or Asian carp.

"If we extrapolate the results of this study across the country and include feral cats, we find that cats are likely killing more than 4 billion animals per year, including at least 500 million birds," says George Fenwick, president of the American Bird Conservancy, in a press release about the study. "Cat predation is one of the reasons why one in three American bird species are in decline."

"I think it will be impossible to deny the ongoing slaughter of wildlife by outdoor cats given the videotape documentation and the scientific credibility that this study brings," adds Michael Hutchins, executive director and CEO of the Wildlife Society. "There is a huge environmental price that we are paying every single day that we turn our backs on our native wildlife in favor of protecting non-native predatory cats at all costs, while ignoring the inconvenient truth about the mortality they inflict."

See the KittyCams website for photos, videos and data from the study. To get tips on keeping cats indoors, check out Ohio State University's Indoor Pet Initiative or the American Bird Conservancy's Cats Indoors Program. And if you know a cat that just can't be fenced in, you could at least attach a bell to its collar, or even dress it up in a bird-protecting "cat bib." (Fair warning: The cat may then want to kill you instead).

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